How can POST audio file and get response WP8 C#

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C#

Windows

Hi. i'm trying to POST audio file to my Web Service ( ASP.Net 4.5 Web API ) and get Response from server but have error. So please help me fix that

Here my code to POST Audio

byte[] buffer;
        private async void Upload(StorageFile fileStream)
        {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(new Uri("http://sv.volcanosoft.com:47581/api/SpeechRecognition?dataType=json"));
            
            request.Method = "POST";
            request.ContentType = "multipart/form-data";
            request.UseDefaultCredentials = true;

            var file = await fileStream.OpenReadAsync();

            byte[] audioDataBytes;
            audioDataBytes = new byte[file.Size];
            for (int i = 0; i < audioDataBytes.Length; i++)
            {
                audioDataBytes[i] = (byte)file.AsStreamForRead().ReadByte();
            }

            const int MAX_URI_LENGTH = 32766;
            string base64audio = System.Convert.ToBase64String(audioDataBytes);
            StringBuilder sb = new StringBuilder();

            for (int i = 0; i < base64audio.Length; i += MAX_URI_LENGTH)
            {
                sb.Append(Uri.EscapeDataString(base64audio.Substring(i, Math.Min(MAX_URI_LENGTH, base64audio.Length - i))));
            }

            string uploadRequestString = sb.ToString();

            buffer = Encoding.UTF8.GetBytes(uploadRequestString);

            request.BeginGetRequestStream(new AsyncCallback(ReadCallback), request);
        }

        private void ReadCallback(IAsyncResult asynchronousResult)
        {
            HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;
            // End the operation.            
            Stream postStream = request.EndGetRequestStream(asynchronousResult);
            postStream.Write(buffer, 0, buffer.Length);
            postStream.Close();
            request.BeginGetResponse(new AsyncCallback(ResponseCallback), request);
        }

        private void ResponseCallback(IAsyncResult asynchronousResult)
        {
            HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;
            HttpWebResponse resp = (HttpWebResponse)request.EndGetResponse(asynchronousResult);
            Stream streamResponse = resp.GetResponseStream();
            StreamReader streamRead = new StreamReader(streamResponse);
            string responseString = streamRead.ReadToEnd();
            Dispatcher.BeginInvoke(new Action(() => { MessageBox.Show(responseString); }));
            // Close the stream object.            
            streamResponse.Close();
            streamRead.Close();
            // Release the HttpWebResponse.            
            resp.Close();
        }

And here my error on Server

Invalid 'HttpContent' instance provided. It does not have a 'multipart' content-type header with a 'boundary' parameter.
Parameter name: content